The present invention relates to a dispersion compensating optical fiber which is connected to an optical fiber having positive dispersion and has negative dispersion so as to make the chromatic dispersion of the optical fiber almost zero.
An optical fiber which is generally installed as an optical communication transmission path at present is a single-mode optical fiber which exhibits zero dispersion at a wavelength of 1.3 .mu.m. In order to increase the capacity of an optical communication network constituted by single-mode optical fibers which are generally installed, it is discussed to perform high-speed communication with light having a wavelength of 1.55 .mu.m.
A single-mode optical fiber which has zero dispersion at a wavelength of 1.3 .mu.m exhibits a positive dispersion with a value of about 17 ps/nm/km near a wavelength of 1.55 .mu.m. For this reason, light having a wavelength of 1.55 .mu.m is used, a dispersion compensating means is required.
As a practical method for dispersion compensation, a method of inserting a great negative dispersion optical fiber into an optical fiber transmission path to cancel the dispersion out. This method uses a phenomenon that a negative structure dispersion having a large absolute value can be obtained by properly selecting the refractive index distribution of an optical fiber.
One concrete dispersion compensating structure using this method is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 6-11620. A dispersion compensating optical fiber disclosed in this publication, which has a great negative dispersion, has the same structure as an ordinary optical fiber except for refractive index. When this dispersion compensating optical fiber is only inserted into an optical fiber transmission path in serial, dispersion compensation can be easily performed to chromatic dispersion of the single-mode optical fiber.
Since the dispersion compensating optical fiber is considerably long, in general, the optical fiber is wound into a coil and then packaged to be used. When the optical fiber is incorporated in a transmission apparatus, the optical fiber is preferably packaged with a small size.
However, a long optical fiber is wound, the space occupied by the coil depends on the volume of the optical fiber sectional area.times.length and the percentage of void. For this reason, when the sectional area, i.e., diameter, of the optical fiber is large, a coil having a large size is obtained. The coil cannot be stored within the transmission apparatus.
As another problem, polarization mode dispersion is posed. When the winding diameter of the optical fiber is decreased, the polarization mode dispersion increases in inverse proportional to the square of the winding diameter.
The dispersion compensating optical fiber has as its object to compensate for the chromatic dispersion of a single-mode optical fiber and to make it possible to perform high-speed communication. However, when polarization mode dispersion increases, even if chromatic dispersion is compensated for, the waveform of a transmission signal is disturbed by polarization mode dispersion. As a result, a reduction in size of a dispersion compensating optical fiber is obstructed.